1. Field of the Invention
The invention relates to a crystalline silicon ingot and a fabricating method thereof, and particularly to a crystalline silicon ingot having a low bulk defect density and small-sized silicon grains at the bottom thereof by using a nucleation promotion layer and a fabricating method thereof.
2. Description of the Prior Art
Most of the solar cells produce a photovoltaic (PV) effect when absorbing sunlight. Currently, the solar cell is made of a silicon-based material, since for the most parts, silicon is the second most abundant and accessible element in the world. Also, silicon is cost-effective, nontoxic, and chemically stable, and becomes broadly used in semiconductor applications.
There are three kinds of silicon-based materials for fabricating solar cells, i.e., single-crystal silicon (mono-Si), polysilicon (poly-Si), and amorphous silicon (a-Si). Poly-Si is much less expensive than mono-Si when produced by Czochralski method (CZ method) or floating zone method (FZ method), so it is usually used as a raw material of the solar cell due to the economic concern.
Conventionally, poly-Si for solar cells is fabricated by a common casting process. That is, it is prior art to produce poly-Si for solar cells by a casting process. In brief, the poly-Si solar cell is fabricated by melting high purity silicon in a mold like quartz crucible, then cooling with a controlled solidification to form a poly-Si ingot, followed by slicing it into wafers that fit compactly into a PV cell module for further application. The ingot formed by the above process is in fact in the form of an aggregation of silicon crystals having random crystal orientations.
It's difficult to texture (roughen) the surface of the poly-Si chip due to the random crystal orientations of grains. Surface texturing can enhance the efficiency of the PV cell by reducing light reflection and thus increasing solar energy absorption on the surface of the cell. In addition, “kinks” that form in the boundaries between the grains of conventional multi-crystalline silicon tend to nucleate structural defects in the form of clusters or lines of dislocations. These dislocations, and the impurities tended to be attracted by dislocations, are believed to cause a fast recombination of electrical charge carriers in a photovoltaic cell made from conventional multi-crystalline silicon, reducing the power output of the solar cell. Thus, the poly-Si PV cell usually has lower efficiency than the equivalent mono-Si PV cell, even a radial distribution of defects exists in the latter manufactured by the current technique. However, because of the relatively simple fabricating process and lower cost for the poly-Si solar cell and also the effective defect passivation step in processing the solar cell, poly-Si is still more broadly used as the silicon source of the PV cell.
Currently, it has been developed that crystalline silicon ingot is fabricated using a mono-Si seed layer and based on directional solidification, in which a large-sized, (100)-oriented mono-Si cubic is generally employed as a seed. Unfortunately, during the competition among the (100)-oriented grain and the random nucleation grain, the latter is prevailing. For maximizing the seeded crystalline volume in an ingot, the current technique takes advantage of the boundaries in (111)-oriented silicon to surround the regions occupied by the (100)-oriented silicon seeds, thereby impeding successfully the growth of crystals having other orientations. In this way, a high quality ingot of mono-Si or bi-crystal silicon block may be obtained, in which the lifetime of the minority charge carriers is maximized in the resultant wafer employed for fabricating the high-performance solar cell. Herein, the term “single crystal silicon (mono-Si)” is referred to a bulk of mono-Si that has a single uniform crystal orientation throughout the bulk, while the term “bi-crystal silicon” is referred to a silicon bulk that has one uniform crystal orientation in or over 50% of the volume of the bulk, and has another uniform crystal orientation in the rest of the volume of the bulk. For example, such bi-crystal silicon may include a body of single crystal silicon having one crystal orientation next to another body of single crystal silicon having a different crystal orientation making up the balance of the volume of crystalline silicon. Additionally, conventional multi-crystalline silicon refers to crystalline silicon having cm-scale grain size distribution, with multiple randomly oriented crystals located within a body of silicon. However, the crystalline silicon ingot fabricated by the current technique described above where the expensive mono-Si is used as a seed is rather costly.
There are other techniques without using expensive mono-Si as a seed. Laterally grown crystals are spread over the bottom of the crucible by partial undercooling first, and then columnar crystals are grown upwards. The large-sized silicon grains of thus obtained ingots have a low bulk defect density. Therefore, the solar cell made from silicon wafers sliced from the crystalline silicon ingot fabricated by the above techniques may have higher photoelectric conversion efficiency.
However, the above current techniques using poly-Si are only proved successful in the laboratory, while in an industrial level production, it's usually more difficult to perform the poly-Si casting by controlling the growth of the dendrites to be spread over the bottom of the crucible using partial undercooling. Industrial-scale multi-crystalline silicon casting is affected by the heating uniformity of the crucible and the entirety, which increases variance of the initial undercooling controlling. Therefore, the poly-Si at the bottom of the crucible tends to grow into a large-sized grain and the defect density in this area will become elevated. The defect density becomes higher rapidly as the large-sized grains proceed to grow, resulting in poor quality of the entire crystalline silicon ingot and the solar cell with reduced photoelectric conversion efficiency.